The family of a man who allegedly plotted to blow up a weapon of mass destruction in the US said they were stunned by his arrest.

A man arrested trying to blow up the US Federal Reserve building in New York is the son of a bank manager, reports say.

Quazi Mohammad Rezwanul Ahsan Nafis, of Bangladesh, has been charged with trying to detonate a fake device in a vehicle parked outside the Manhattan building, home to "the world's largest accumulation of gold".

In an elaborate FBI sting, agents grabbed Nafis when he made several attempts to detonate the 1,000lb device by mobile phone.

Nafis had travelled to the US with "the purpose of conducting a terrorist attack" and actively tried to make al Qaeda contacts after his arrival, authorities said.

But the family of the 21-year-old suspect insist he had never displayed any radical tendencies and said he was a devout Muslim whose arrest had come as a shock.

His father, Quazi Mohammad Ahsanullah, said: "We're stunned. Nafis is not a radical type. He says prayers five times a day, and reads the holy Koran and Hadith every day.

"I have never seen him reading any books on jihad. We don't believe that he can have committed this... He is our pride and joy."

Nafis' extended family lives in North Jatrabari, an upper middle-class neighbourhood of southeastern Dhaka. His father is a senior vice president of National Bank and his sister is a doctor.

The bank is one of the most fortified buildings in the city
Speaking from the family home, Nafis' brother-in-law Arik said they had spoken to him only hours before his arrest and even discussed a possible bride for him.

"We heard the news this morning. Everyone is crying here," Arik told AFP. "Nafis never showed any form of radicalisation when he was in Bangladesh."

Officials at North South University in Bangladesh said Nafis had struggled during his eight terms as an electrical engineering and telecommunications student and had been effectively forced to leave after disappointing exam results.

His family said he then moved to the US where he initially took up a place at Missouri Southern State University, left after a term due to cost and took a job at a New York hotel.

Authorities insisted the alleged terror plot never posed an actual risk - but that it demonstrated the value of using sting operations to neutralise young extremists eager to harm Americans.

Before trying to carry out the plot, Nafis went to a warehouse to help assemble a 453kg (1,000lb) bomb using inert material, according to the FBI.

Court documents claim he asked an undercover agent to videotape him saying: "We will not stop until we attain victory or martyrdom."

An official complaint said Nafis contacted a confidential FBI informant in July telling him he wanted to form a terror cell.

In further conversations, authorities said Nafis proposed several spots for his attack, including the New York Stock Exchange, and that in a written letter taking responsibility for the Federal Reserve job he was about to carry out, he said he wanted to "destroy America".